14
MODBUS RTU Transmission Mode
When the instrument uses RTU (Remote Terminal Unit) mode for MODBUS serial communication, each
8-bit byte of information contains two 4-bit hexadecimal characters. The main advantages of this
mode are greater character density and better data throughput than the ASCII mode with the same
baud rate. Each message must be transmitted as a continuous string.
The format of each byte in RTU mode (11 bits):
Coding system: 8-bit binary
Each 8-bit byte in a message contains two 4-bit hexadecimal characters (0-9, A-F)
Bits in each byte: 1 starting bit
8 data bits, the first minimum valid bits without parity check bits
2 stop bits
Baud rate: 9600 BPS
How characters are transmitted serially:
Each character or byte is sent in this order (from left to right) the least significant bit (LSB)... Maximum
Significant Bit (MSB)
Figure 3: RTU pattern bit sequence
Check Domain Structure:
Cyclic Redundancy Check (CRC16)
Structure description:
Figure 4: RTU information structure
The maximum frame size of MODBUS is 256 bytes
MODBUS RTU Information Frame
In RTU mode, message frames are distinguished by idle intervals of at least 3.5 character times, which
are called t3.5 in subsequent sections.
Figure 5: RTU message frame
The entire message frame must be sent in a continuous character stream.
When the pause time interval between two characters exceeds 1.5 characters, the information frame
is considered incomplete and the receiver does not receive the information frame.
MODBUS RTU CRC Check
The RTU mode contains an error-detection domain based on a cyclic redundancy check (CRC)
algorithm that performs on all message contents. The CRC domain checks the contents of the entire
message and performs this check regardless of whether the message has a random parity check. The
CRC domain contains a 16-bit value consisting of two 8-bit bytes. CRC16 check is adopted..Low bytes
precede, high bytes precede.
Start bit 1 2 3 4 5 6 7 8 Stop bit Stop bit
Slave Instrument
Address
Function Code
1 byte
Data
0…252 byte
CRC
2 byte
CRC Low byte CRCHigh byte
Starting
≥
3.5 characters
Address
8 bit
Function code
8 bit
Data
Nx8 bit
CRC check
16 bit
End
≥
3.5 characters
At least 3.5 characters
At least 3.5 characters
Frame 1
Frame 2
Frame 3
4.5 characters
3.5 characters
Frame 1 normal
≤
1.5 characters
＞
1.5 characters
Frame 2 failure
